(1) Field of the Invention
This invention concerns bond-ply materials comprising at least one resin and at least one nonwoven reinforcing material. The present invention also includes methods for using the bond-ply materials of this invention to manufacture high density multilayer printed wiring boards.
(2) Description of the Art
The electronics industry continues to seek enhanced product performance in order to meet consumer demands for higher functionality and lower cost computers and other electronic devices. Among the methods that the electronic industry is using to increase performance is by designing of multilayer circuit boards that have smaller, finer circuit lines and spaces. Increasing line density leads to fewer circuit layers resulting in smaller electronic devices. The methods for fabrication of multilayer boards are well-known in the art. One commonly used method is to use a joint layer to unite two separate circuit layers. Such joint layer is known in the art as prepreg or bond-ply. Through holes or vias are drilled in the bond-ply and metallized to form electrical interconnections between the circuit layers.
Recently, the industry began migrating towards micro-via technologies to join and electrically interconnect multiple circuit layers. The use of micro-vias, typically smaller than 150 microns in diameter, significantly densifies the circuitry due to reduced amount of board area needed per via. Lasers are used in the printed wiring board industry to drill micro-vias. The methods for using lasers to drill micro-vias in electronic substrates such as films and nonwoven reinforced laminates have been described in the prior art.
The configuration and properties of a bond-ply material significantly affect the fabrication and performance of a multilayer printed wiring board. With increasing circuit density and finer circuit traces it is becoming more important to use bond-plys in the fabrication of high density multilayer wiring boards that have good dimensional stability and that behave uniformly when subjected to environmental stress over dimensions comparable to or smaller than the dimensions of the circuitry to facilitate registration of the vias in the bond-ply with the pads in the circuit layers. Furthermore dimensional uniformity of the bond-ply is also required in order to assure that warping or buckling does not occur after processing and circuit fabrication and assembly. In general it is known that fiber reinforcement moderate of the relatively high thermal coefficient of expansion of polymeric materials, that it provides for mechanical rigidity, and that fiber reinforcement can reduce the brittleness of composite materials. It is also important that the bond-ply can be consistently laser drilled to form micro-vias as mechanical drilling of microvias is inefficient and expensive. Furthermore, it has become important to have a uniform thickness of dielectric material between the circuit layers that are joined by a bond-ply. In general, however, prior art reinforced bond-ply materials are either not sufficiently uniform to prevent warp and buckle or cannot be laser drilled to form uniform micro vias. Prior art reinforced bond-ply materials also are generally insufficiently uniform to provide a consistent and known final dielectric thickness between circuit layers after joining them.
Despite the advancements made in the quality of electronic substrates, there remains a need for improvements. Specifically, there remains a need for a bond-ply that cleanly ablate when lased to produce quality through holes or vias. There also remains a need for reinforced bond-plies with improved dimensional stability and strength. And their remains a need for bond-plies that can be efficiently and effectively used to manufacture high density multilayer wiring boards. There also remains a need for a bond-ply that provides for a uniform and known dielectric thickness between two circuit layers after joining the layers together.
An object of the present invention is to provide a bond-ply material that has a coefficient of thermal expansion (CTE) that is closely matched to the CTE""s of circuit layers associated with the material, that is resistant to moisture absorption, and that has high heat resistance.
Another object of the present invention is to provide a bond-ply material that, after curing, has a high degree of dimensional stability and uniformity when subjected to changing environmental conditions such as temperature and humidity.
Yet another object of the present invention is to provide a bond-ply material that has sufficient resin flow during lamination to fill the spaces between circuit traces yet has a consistent and controlled dielectric spacing between circuit layers.
Still another object of the present invention is to provide a bond-ply material that is amenable to laser via fabrication.
Another object of the present invention is to provide a bond-ply material that creates a uniform and predictable dielectric thickness between circuit layers which it bonds together.
The present invention includes a bond-ply sheet containing a nonwoven reinforced C-staged (predominantly cured) core layer having a first surface and a second surface, a first B-staged (partially cured) resin layer covering the core layer first surface, a second B-staged resin layer covering the core layer second surface, and optionally a release film or layer that covers one or both of the B-stage coatings. The core of the Bond-ply preferably exhibits a thermal expansion coefficient of between 2 and 20 parts per million per Centigrade after curing.
The present invention also includes a bond-ply sheet containing a nonwoven reinforced, partially cured core layer having a first surface and a second surface, a first B-staged resin layer covering the core layer first surface, a second B-staged resin layer covering the core layer second surface, and optionally a release film or layer that covers one or both of the B-staging coatings. The core of the bond-ply preferably exhibits a thermal expansion coefficient of between 2 and 20 parts per million per Centigrade after curing.
The present invention further includes a bond-ply sheet including a nonwoven reinforcing material comprising a mixture of micro-glass fibers and organic fibers or pulps, a B-staged resin, and optionally a release film or layer that covers one or both of the coatings.
The present invention still further includes bond-ply sheets of this invention including at least one microvia hole perpendicular to the plane of the bond-ply sheet, which is filled with an electrically-conductive filler, such as paste or a thermally activated precursor to an electrically-conducting material.
The present invention further includes methods for interconnecting high density electronic circuit layers to manufacture multilayer boards with the bond-ply materials of this invention comprising the steps of drilling holes within the bond-ply material of this invention using a laser or other means, filling the holes with conductive paste or conductor precursor, optionally removing the release film or layers from the bond-ply material, inserting bond-ply material between two planar electronic circuit elements such that the electrically-conductive paste or precursor contacts electrically-conductive circuit regions upon the circuit elements, subjecting the structure to pressures between 0 and 1000 psi and temperature of from 25 to about 400xc2x0 C. to cause the polymeric coating materials to flow, and optionally subjecting the structure to temperatures and pressures sufficiently high to cure the thermosetting polymers or the thermosetting adhesives and/or to activate the electrically conducting precursor if necessary.